OFDMA
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique used at the physical layer (PHY) of a number of wireless networks, e.g., networks designed according to the well known IEEE 802.11a/g and IEEE 802.16/16e standards. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme based on OFDM. In OFDMA, separate sets of orthogonal tones (subchannels or frequencies) and time slots are allocated to multiple transceivers or mobile stations (MS) by a base station (BS) so that the transceivers can communicate concurrently. OFDMA is widely adopted in many next generation cellular networks such as networked based on 3GPP Long Term Evolution (LTE), and IEEE 802.16m standards due to its effectiveness and flexibility in radio resource allocation.
OFDMA Resource Allocation
Time and frequencies in the radio spectrum are scarce resources in wireless communications, and therefore an efficient allocation is needed. The rapid growth of wireless applications and subscriber transceivers, i.e. mobile stations (MS), have require a good radio resource management (RRM) scheme that can increase the network capacity and reduce deployment costs. Consequently, developing an effective radio resource allocation scheme for OFDMA is of significant interest for wireless communication.
The fundamental challenge is to allocate the limited available spectrum in a large geographical for a large number of transceivers. Typically, the resources are allocated by base stations (BS). In other words, the same frequency spectrum can be used in multiple geographical areas or cells. This will inevitably cause inter-cell interference (ICI), when transceivers or mobile stations (MSs) in adjacent cells use the same spectrum. In fact, ICI has been shown to be the predominant performance-limiting factor for wireless cellular networks.
To maximize the spectral efficiency, a frequency reuse factor of one is used in OFDMA cell deployment, i.e., the same spectrum is reused by each BS and MS in each and every cell. Unfortunately, this high spectrum efficiency also unavoidably leads to ICI. Therefore, a good ICI management scheme is needed.
For a single cell, most of existing allocation methods optimize power or throughput under an assumption that each MS uses different subchannel(s) in order to avoid intra-cell interference. Another key assumption in single-cell resource allocation is that the BS has signal-to-noise ratios (SNR) for all channels. In a downlink (DL) channel from the BS to the MS, the SNR is normally estimated by the MS and fed back to the BS. In the uplink channel from MS to BS, the BS can estimate the SNR directly based on the signal received from the BS.
In a multi-cell scenario, the signal-to-interference-and-noise ratio (SINR) is difficult to obtain because the interference can come from multiple cells and depends on a variety of factors, such as distance, location, and occupied channel status of interferers, which are unknown before resource allocation. This results in mutual dependency of the ICI and complicates the resource allocation problem. Thus, a practical multi-cell resource allocation method that does not require global and perfect knowledge of SINR is desirable.
Inter-Cell Interference Coordination (ICIC)
ICIC is a technique that can effectively reduce ICI in regions of cells relatively far from the BS. ICIC is achieved by allocating disjoint channel resources to the MSs near the boundary of the cell that are associated with different cells. Because boundary MSs are most prone to high ICI, the overall ICI can be substantially reduced by coordination of channel allocation among boundary MSs. More specifically, the ICIC reduces ICI interference by allocating the same resource to MSs that geographically far apart MSs so that path loss due to the interference is reduced.
However, ICIC solely based on avoiding resource collision for boundary MSs only offers a limited performance gain for DL communications, because it does not consider interference caused by transmission from the BS to cell-center MSs.
Spatial Division Multiple Access (SDMA)
Space division multiple access (SDMA) provides multi-transceiver channel access by using multiple-input multiple-output (MIMO) techniques with precoding and multi-transceiver scheduling. SDMA exploits spatial information of the location of the MSs within the cell. With SDMA, the radiation patterns of the signals are adapted to obtain a highest gain in a particular direction. This is often called beam forming or beam steering. BSs that support SDMA transmit directed signals to multiple transceivers concurrently using the same resources. Thus, SDMA can increase network capacity.
Base Station Cooperation (BSC)
Base station cooperation (BSC) allows multiple BSs to transmit signals to multiple MSs concurrently while sharing the same resource, i.e., time and frequency. BSC utilizes the SDMA technique for the BSs to send signals to the MSs cooperatively, and is specifically used for boundary MSs that are within the transmission ranges of multiple BSs. In this case, the interfering signal becomes part of a useful signal. Thus, BSC has two advantages, spatial diversity and ICI reduction.
Diversity Set
Typically, each MS is ‘registered’ at and communicates with one BS, which is called the anchor or serving BS. However, in some scenarios such as handover, concurrent communication with multiple BSs can take place. A diversity set is defined in the IEEE 802.16e standard to serve this purpose. The diversity set keeps track of the anchor BS and adjacent BSs that are within the communication range of a MS. The information of the diversity set is also maintained and updated at the MS.
Graph-Based Framework
The channel assignment problem in conventional (non-OFDMA) cellular and mesh networks has been solved using a graph coloring approach. In the conventional problem formulation, each node in the graph corresponds to a BS or an access point (AP) in the network to which channels are allocated. The edge connecting two nodes represents the potential co-channel interference, which typically corresponds to the geographical proximity of the BSs. Then, the channel assignment problem that respects the interference constraints becomes the graph coloring problem, where nodes representing two interfering base stations should not have the same color, i.e., use the same channel.
In conventional networks, if two adjacent base stations transmit at the same time using the same spectrum, then they cause interference to each other in the MSs. Thus, in the conventional graph, all that is required is to ensure that adjacent nodes representing base stations have different colors. That solution is however inapplicable to OFDMA networks, where the frequency reuse factor is one, and all BSs do use the same spectrum. In addition, conventional graphs do not consider technologies such as ICIC and BSC, as described above.